Diagnostic errors in ambulatory care present a vexing challenge, at least in part because of limited measurements to identify events that unfold over time and space (various healthcare interactions). Although the NAM report emphasizes the patient perspective, traditional diagnostic error measures do not. New data suggest that patient experiences of diagnostic error reflect themes virtually absent from current metrics: ignoring patient knowledge, disrespect, and other unprofessional behavior and communication barriers. Lack of patient-centered metrics truncates organizational learning and the ability to more meaningfully prevent future diagnostic errors, which levy an enormous burden on patients -- particularly the sickest children and adults. Fortunately, as health information transparency spreads, there are new ways to engage patients in the diagnostic process. Sharing visit notes with patients through the patient portal provides them with a written record of their ambulatory experiences, and represents a rich but relatively untapped existing source for patients to help identify breakdowns in diagnosis and care. Growing evidence demonstrates the unique knowledge of patients and their ability to identify such breakdowns. Over 40 million patients across the US can now access open notes. Studies demonstrate patient safety and engagement benefits from open notes, especially among more vulnerable patients. Working firsthand on implementation and measurement of the effects of shared visit notes over the last decade, our team has gained a nuanced understanding of opportunities and challenges related to transparent notes in virtually every medical field. We have also innovated two new approaches, enabling patients to a) contribute to notes and b) to report on potential note errors. Our research shows frequent patient-reported breakdowns related to the diagnostic process. Partnering with patients/families, health services researchers, and diagnostic error experts, we are now uniquely poised to: 1a) Establish a new patient-centered framework co-designed with patients/families and care partners to measure and categorize patient-reported diagnostic breakdowns (PRDBs), and b) apply this new analytic tool to establish the incidence, types, and contributing factors to PRDBs in ambulatory care using 2 large and unique existing databases; 2) Develop and implement a new EHR portal- based method enabling chronically ill patients and their families to a) contribute to the visit note and diagnostic process and b) identify and report diagnostic breakdowns using existing EHR data; and 3) Assess the use and impact of this method on the diagnostic process measuring safety (incidence/types of PRDBs among chronically ill patients), implementation, and stakeholder experience outcomes. Our findings will improve our understanding of ambulatory diagnostic breakdowns and establish a new method for organizations to systematically partner with patients in the diagnostic process. Study results will provide the foundation to ultimately design and test interventions to reduce ambulatory diagnostic errors.